Benchmark inject() versus each_with_object()

It was pointed out by jhawthorn and
zenspider in a Reddit thread that some of my
inject() examples from this
post could be improved by
simply using each() or each_with_object().

I took the original function I had used (which has since been replaced with a
slightly improved variant, still using inject()). I then rewrotie it with a
small internal optimization for inject(), using just each() and then
each_with_object().